1. Field of the Invention
The present invention relates to digital cameras, in particular to digital cameras that provide an electromechanical technique for exposure control, i.e., exposing all pixels of an image substrate simultaneously when desired.
2. Description of the Related Art
The human eye perceives natural visible light using an analog process. Existing electronic technology cannot duplicate this natural phenomenon, however, ironically, electronically communicating natural light signals via a digital process has recently been found to have advantages over electronically communicating natural light signals using existing analog techniques. Specifically, video is now making a conversion to digital technology for acquisition, storage, and communication. For example, a CCD (charge-coupled device) camera provides for image acquisition, digital transmission of the images, video compression, high density storage of the image on a video compact disk, and/or for video conference calls with images.
A preferred interface to digital video is a digital display. For example, conventional digital projection display technology accepts digital video and transmits to the eye a burst of digital light pulses that the eye interprets as an analog color image. Microelectromechanical systems (MEMS) devices known as Digital Micromirror Devices (DMDs), trademark of Texas Instruments, is a fast, reflective digital light switch. A DMD may be combined with image processing, memory, a light source, and optics to form a digital system capable of projecting large, bright, seamless, high-contrast color images with better color fidelity and consistency than displays of the past. The fabrication of the DMD superstructure begins with a completed CMOS (complementary memory oxide semiconductor) memory circuit. Through the use of six photomask layers, the superstructure is formed with alternating layers of aluminum for the address electrode, hinge, yoke, and mirror layers and hardened photoresist for the sacrificial layers that form the two air gaps. The aluminum is sputter-deposited and plasma-etched. The sacrificial layers are plasma-etched to form the air gaps.
A DMD may be described as a semiconductor light switch. Thousands of thin, square mirrors, fabricated on hinges atop a static random access memory (SRAM) make up a DMD. Each mirror is capable of switching a pixel of light. The hinges allow the mirrors to tilt between two states, +10 degrees for “on” or −10 degrees for “off.” When the mirrors are not operating, they sit in a “parked” state at 0 degrees. Each mirror in a DMD system is electrostatically tilted to the on or off positions. The technique that determines how long each mirror tilts in either direction is called pulsewidth modulation (PWM). The mirrors are capable of switching on and off more than 1000 times a second. This rapid speed allows digital gray scale and color reproduction. Gray scale is achieved by binary pulsewidth modulation of the incident light. Color is achieved by using color filters, either stationary or rotating, in combination with one, two, or three DMD chips. After passing through condensing optics and a color filter system, the light from the projection lamp is directed at the DMD. When the mirrors are in the on position, they reflect light through the projection lens and onto the screen to form a digital, square-pixel projected image.
Color is added to a DMD system by adding a color wheel to create a full-color projected image. The color wheel is a red, green, and blue filter system that spins at 60 Hz to give 180 color fields per second. The input signal is broken down into RGB (red green blue) data and is sequentially written to the DMD's SRAM. A white light source is focused onto the color wheel through the use of condensing optics. The light that passes through the color wheel is then imaged on to the surface of the DMD. As the wheel spins, sequential red, green, and blue light hits the DMD. The color wheel and video signal are in sequence so that when red light is incident on the DMD, the mirrors tilt ‘on’ according to where and how much red information is intended for display. The same is done for the green and blue lights and video signals. The human visual system integrates the red, green, and blue information and sees a full color image. Using a projection lens, the image formed on the surface of the DMD can be projected onto a large screen. However, when a color wheel is used, two-thirds of the light is blocked at any given time. As white light hits the red filter, the red light is transmitted and the blue and green light is absorbed. The same holds true for the blue and green filters: the blue filter transmits blue and absorbs red and green, the green filter transmits green and absorbs red and blue.
Similar problems exists in other approaches to color reproduction in DMD systems. For example, a three chip system (i.e., three DMD chips) has been implemented where color is added by splitting white light into the three primary colors by using a prism system. One DMD is used for each of the primary colors. With three DMDs, light from each primary color is directed continuously at its own DMD for the entire field. The result is that more light reaches the screen, giving a bright projected image. Another system is the two DMD chip system.
The two DMD chip system takes advantage of the red light deficiency in a metal halide-type projection lamp. The color wheel on this system does not use red, green, and blue filters. Instead, the system uses two of the secondary colors, magenta and yellow. The magenta segment of the color wheel allows both red and blue to pass through while the yellow segment passes red and green. The result is that red light is constantly passing through the filter system. Red is on all the time. Blue and green alternate with the rotation of the magenta-yellow color wheel and are each essentially on half the time. Once through the color wheel, the light is directed to a dichroic prism system. At this point, the constant red light is split off and directed to a DMD that is dedicated to handling red light and red component video signals. The sequential blue and green light is directed to another DMD that is configured to handle the alternating colors. This DMD is driven by blue and green component video signals.
The two DMD chip system architecture is capable of achieving more than three lumens of spectrally balanced light output per watt of input. However, a system that could be implemented without a color wheel would be advantageous for at least the reason that it would be less complex to fabricate.
Many other problems and disadvantages of the prior art will become apparent to one skilled in the art after comparing such prior art with the present invention as described herein.